1. Field of the Invention
The present invention relates to a shift range change system of an automatic transmission of a vehicle.
2. Description of Related Art
For example, one previously proposed automatic transmission of a vehicle includes a shift range change mechanism and a parking change mechanism and is shifted manually by a driver of the vehicle. However, lately, a shift range change system (a shift-by-wire abbreviated as “SBW”), which changes the shift range change mechanism and the parking change mechanism through a rotary actuator having an electric motor, is prevailing in the market.
In general, the vehicle is designed on the assumption that the vehicle is used in various conditions, so that the vehicle is designed to be parked on, for example, a slope (a sloping road).
At the time of parking the vehicle on the slope, the gravitational force, which acts on the vehicle to move the vehicle, is applied through an axle of the vehicle to an engaged part of the parking change mechanism (an engaged part between a parking gear and a park pole) and also between the park pole and a park rod in the parking change mechanism. This force is increased proportional to the tilt angle of the vehicle on the slope. In view of this, the rotary actuator (the SBW actuator) is set to generate a large output torque to smoothly release the engagement of the parking change mechanism at the time of parking release (time of changing from the parking range to the corresponding non-parking range) for releasing the engagement of the parking change mechanism even in the parked state of the vehicle on the slope.
As described above, the rotary actuator is set to generate the large torque, which is required at the time of the parking release on the slope.
Here, a technique of “parking range wall (P-wall) abutment learning” is known. In this P-wall abutment learning, when an ignition (IG) switch is turned on, the electric power is supplied to the motor to abut a movable member of the shift range change mechanism against a parking range side movable limit position thereof to sense a reference position of the rotor of the motor.
However, when the P-wall abutment learning is executed with the maximum torque of the motor (the large torque A, which is required at the time of executing parking release of the vehicle on the slope), a relatively large mechanical load is generated at the time of abutment of the movable member against the stationary member (the rotary actuator and the automatic transmission), so that the mechanical damage may possibly occur in the movable member and/or the stationary member.
For example, JP2004-308752A (corresponding to US2006/0207373), JP2001-271917A, JP2006-191709A (corresponding to US2006/0138880) and JP2007-170545A (corresponding to US2007/0144287) disclose a technique of reducing the output torque of the motor with use of duty ratio control at the time of executing the P-wall abutment learning.
Lately, many shift range change systems have “auto-P function”, which sets the shift range of the automatic transmission to a parking range (P-range) and then turns off the electric power source of the motor control apparatus (SBW ECU). Therefore, at the time of turning on of the IG switch, the shift range of the automatic transmission is the P-range.
However, sometimes, the IG switch is turned on from the other shift range (hereinafter, referred to as a non-parking range), which is other than the P-range. In order to facilitate understanding of this situation, a specific example will be described. At the time of assembling the vehicle at the factory, the range is set to a neutral range (N-range) to enable the transferring of the vehicle from one assembling location to another assembling location. The IG switch may be turned on from that state (i.e., in the N-range). In such a case, the electric power source of the motor control apparatus may be intentionally turned off at the N-range to enable the towing of the vehicle. Thereafter, the IG switch may be turned on from that state. Alternatively, the electric power source of the SBW ECU may be turned off at the non-parking range due to malfunctioning. Thereafter, the IG switch may be turned on from that state (i.e., in the non-parking range).
Here, the P-wall abutment learning is executed as follows. That is, the motor is rotated toward a parking range side, and the movable member of the shift range change mechanism is abutted against the parking range side movable limit position (e.g., is abutted against a wall referred to as the P-wall). The output torque at the time of executing the P-wall abutment learning is determined in view of the case where the IG switch is turned on from the non-parking range (the case occurring at the very low frequency).
That is, at the time of executing the P-wall abutment learning, the duty ratio control is executed to reduce the output of the motor 5. However, according to the previously proposed technique, the duty ratio is set such that an output torque B, which is required to change from the non-parking range to the parking range (parking setting), is generated from the motor.
The above technique may be summarized as follows. That is, at the time of executing the P-wall abutment learning, the duty ratio control is executed such that the torque of the motor is set to the output torque B, which is required for the parking setting. At the time of executing the other control (the other control, which is other than the P-wall abutment), the torque of the motor is set to the relatively large torque A, which is required at the time of parking release on the slope.
The output torque C of the motor, which is required to change from one of the non-parking ranges to another one of the non-parking ranges at the situation other than the parking release and the parking setting, is smaller than the output torques A, B described above.
Furthermore, the output torque D of the motor, which is required to implement the P-wall abutment learning from the P-range, is smaller than the output torque C.
That is, the output torques, which are required at the motor 5, are set according to the type of the change process to achieve the relationship of the output torque A>the output torque B>the output torque C>the output torque D.
As discussed above, the output torque, which is required at the motor at the time of executing the P-wall abutment learning from the P-range, is the output torque D. However, upon consideration of the P-wall abutment learning from the non-parking range, which occurs at the very low frequency, the output torque of the motor at the time of executing the P-wall abutment learning is set to the output torque B through the duty ratio control in the previously proposed technique.
Therefore, in the case of executing the P-wall abutment learning from the parking range, which occurs at the relatively high frequency, the motor may possibly generate an excessively large torque to apply a relatively large mechanical damage to the movable member and the stationary member (e.g., the rotary actuator and the automatic transmission) to reduce the durability thereof.